Allow selection of memory mode for Cumulative Metrics export

[carlosalberto]

When using Cumulative reporting of Metrics, allow selecting whether, upon lack of Metric activity, a) no Metric should be exported or b) Metric should be exported with the last value (gauges) or cumulative value (counters and histograms) - instead of sending an empty Metric (that is, no data points).

This came up in Java as per open-telemetry/opentelemetry-java#1978 as we are sending empty Metrics using Cumulative reporting. See @jmacd's comment

When configuring the SDK for cumulative-reporting-with-memory, it should continue to report the last value (for gauges) and cumulative values (for counters and histograms)... I would argue that this "Memory" behavior is a choice: whether to report no metric or to report an old value, but in either case we should not report an empty Metric.

As mentioned in the related link, OTel Go supports selecting this memory mode.

[andrewhsu]

from the issue triage mtg today, discussed this and choosing the default seems required for ga, but allowing customization via config seems appropriate for after ga.

can we have a separate tracking issue for config and we have this one just track the default behaviour?

[andrewhsu]

assigning to carlos initially to get the issues split up, but can reassign later if more appropriate

[reyang]

Trying to cover this in #1966:

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/supplementary-guidelines.md#aggregation-temporality
• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/supplementary-guidelines.md#memory-management

I think we will not enforce it in the initial Stable release of the Metrics specification.